Edexcel A-Level Chemistry: Organic Chemistry I
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Two halogenoalkanes are warmed separately with aqueous sodium hydroxide:
- Compound J: 1-bromobutane, CH₃CH₂CH₂CH₂Br (a primary halogenoalkane)
- Compound K: 2-bromo-2-methylpropane, (CH₃)₃CBr (a tertiary halogenoalkane)
Compound J reacts by an SN2 mechanism and compound K reacts by an SN1 mechanism.
Describe the SN2 mechanism for the hydrolysis of compound J and the SN1 mechanism for the hydrolysis of compound K, and explain why each substrate follows its respective pathway.
In your answer refer to the nucleophile, the type of bond fission, any intermediate or transition state, the rate equation for each mechanism, and the role of carbocation stability and steric hindrance.
(6 marks)
A student prepared 1-bromobutane by refluxing butan-1-ol with sodium bromide and concentrated sulfuric acid. The equation for the overall conversion can be written as:
CH₃CH₂CH₂CH₂OH + HBr → CH₃CH₂CH₂CH₂Br + H₂O
The student recorded the following data.
| Quantity | Value |
|---|---|
| Mass of butan-1-ol used | 14.8 g |
| Mass of pure 1-bromobutane collected | 19.2 g |
| Molar mass of butan-1-ol (CH₃CH₂CH₂CH₂OH) | 74.0 g mol⁻¹ |
| Molar mass of 1-bromobutane (CH₃CH₂CH₂CH₂Br) | 137.0 g mol⁻¹ |
| Molar mass of water (H₂O) | 18.0 g mol⁻¹ |
(a) Calculate the percentage yield of 1-bromobutane. Show your working. (3 marks)
(b) Calculate the percentage atom economy for the formation of 1-bromobutane in this reaction. (2 marks)
(c) Suggest one reason, other than losses during transfer or purification, why the percentage yield is below 100 %. (1 mark)
A halogenoalkane L has the structural formula CH₃CH(CH₃)CHClCH₂CH₃ (3-chloro-2-methylpentane).
(a) Identify, by its position number, the chiral centre in molecule L, and state the four different groups attached to it. (2 marks)
(b) A single enantiomer of L is optically active, but the product mixture obtained when 2-methylpentane is chlorinated at that position is optically inactive. Explain both observations. (3 marks)
The unsymmetrical alkene 2-methylbut-2-ene, CH₃C(CH₃)=CHCH₃, reacts with hydrogen chloride by electrophilic addition. Two isomeric chloroalkanes could in principle form, but one is produced in much greater amount.
(a) Give the structural formula and IUPAC name of the major product, and identify the type of carbocation through which it forms. (3 marks)
(b) Explain why this product predominates over the alternative chloroalkane. (2 marks)
The branched alkane 2,3-dimethylbutane, (CH₃)₂CHCH(CH₃)₂, reacts with chlorine in the presence of ultraviolet light. The reaction can replace one hydrogen atom with a chlorine atom (monochlorination).
(a) State why only two different monochloro structural isomers can form, and give the IUPAC name of each. (3 marks)
(b) Write the two propagation steps for the formation of the isomer in which chlorine replaces a hydrogen on one of the CH₃ groups. (1 mark)
(a) Give the IUPAC name of the compound with the structural formula CH₃CH₂CHBrCH(CH₃)₂. (1 mark)
(b) The alkene 3-methylpent-2-ene, CH₃CH=C(CH₃)CH₂CH₃, shows E/Z stereoisomerism. Using the CIP priority rules, state the higher-priority group on each carbon of the C=C double bond, and hence explain how the E and Z isomers are distinguished. (2 marks)